During various orthopedic surgical procedures, it is often necessary to remove small layers of cortical bone. Several different tools have been developed to accomplish this task and for preparing and/or shaping bone surfaces. For example, mallets are often used to apply an impacting force on a medical tool, such as a chisel, to remove pieces of bone. While mallets are somewhat effective, the impacting force must be carefully applied to avoid removal of too much bone or the inadvertent removal of a wrong piece of bone. Moreover, the force applied to the chisel must be applied in a sufficiently accurate manner to avoid damage to adjacent tissues and/organs.
Other surgical tools known as burrs have also been developed for removing layers of cortical bone and shaping bone and cartilage. Such devices, however, generally must be employed with high levels of precision to ensure that only the desired amount of bone is removed and the surrounding tissues are not undesirably damaged or traumatized. These burrs and similar instruments, however, do not provide a means for controlling bleeding and tend to leave the treated tissue with a roughened surface. In an effort to address those problems, radio frequency-based devices were developed.
Radio frequency-based devices enable surgeons to remove, modulate, or sculpt soft tissue while simultaneously sealing blood vessels. They work particularly well on connective tissue, which is primarily comprised of collagen and which contracts when contacted by heat. However, such radio frequency-based devices can create undesirable deep thermal injury in the tissue.
Other instruments that have been developed for effectively cutting and coagulating organic tissue employ mechanical vibrations that are transmitted to a surgical end-effector at ultrasonic frequencies. Ultrasonic vibrations, when transmitted to organic tissue at suitable energy levels and using a suitable end-effector, may be used to cut, dissect, elevate or cauterize tissue or to separate muscle tissue off bone. Ultrasonic instruments utilizing solid core technology are particularly advantageous because of the amount of ultrasonic energy that may be transmitted from the ultrasonic transducer, through a waveguide, to the surgical end-effector.
Activating or exciting the end-effector (e.g., cutting blade) of such instruments at ultrasonic frequencies induces longitudinal vibratory movement that generates localized heat within adjacent tissue, facilitating both cutting and coagulation. Because of the nature of ultrasonic instruments, a particular ultrasonically actuated end-effector may be designed to perform numerous functions, including, for example, cutting and coagulation.
Ultrasonic vibration is induced in the surgical end effector by electrically exciting a transducer, for example. The transducer may be constructed of one or more piezoelectric or magnetostrictive elements in the instrument hand piece. Vibrations generated by the transducer section are transmitted to the surgical end-effector via an ultrasonic waveguide extending from the transducer section to the surgical end-effector. The waveguides and end-effectors are designed to resonate at the same frequency as the transducer. Therefore, when an end-effector is attached to a transducer the overall system frequency is the same frequency as the transducer itself. Nevertheless, those skilled in the art will appreciate that the system may be designed where the transducer and the blade resonate at different frequencies and when joined the system resonates at a desired frequency.
The amplitude of the longitudinal ultrasonic vibration at the tip, d, of the end-effector behaves as a simple sinusoid at the resonant frequency as given by:d=A sin(ωt)where:    ω=the radian frequency which equals 2π times the cyclic frequency, f and    A=the zero-to-peak amplitude.The longitudinal excursion is defined as the peak-to-peak (p-t-p) amplitude, which is just twice the amplitude of the sine wave or 2A.
Over the years, a variety of different ultrasonic blade configurations have been developed. Blades that tend to work well from a coagulation standpoint (and hence change tissue into a sticky coagulum that can be readily reshaped) do not tend to cut extremely well. Some of those blades generally have spherically-shaped body with a substantially smooth outer surface. FIGS. 2 and 3 depict a spherically-shaped blade 10 of this type that has been used in the past. Such blade design, while effective from a coagulation standpoint, is not particularly well-suited for bone removal or tissue reshaping applications due to its shape. Other existing blades that are better adapted for cutting tissue, are not as well-suited to coagulate and reshape tissue. These problems can be further exacerbated in arthroscopic procedures that afford limited access to the target tissue or bone and where the blade must work in an aqueous environment.
It would, therefore, be advantageous to design a harmonic surgical instrument for shaping either soft tissues such as cartilage or meniscus or for decorticating bone. It would be further advantageous to design a harmonic surgical instrument that can be used to decorticate and aspirate bone and also facilitate spot coagulation of tissue as well as tissue reshaping. Various embodiments of the present invention incorporate improvements to known ultrasonic instruments to provide these advantages. The foregoing discussion is intended only to illustrate some of the shortcomings present in the field of the invention at the time, and should not be taken as a disavowal of claim scope.